For many years there has been active and increasing interest in conducting chemical processing in aqueous media. In many cases, a potential feedstock is produced along with water. This occurs, for example, in the commercial maleic anhydride process. Alternatively, in the case of fermentations, potential feedstocks are themselves produced in water. Removal of water from these compositions would be time-consuming and costly. Additionally, water has many advantages over more conventional solvents that present problems with toxicity and difficulties with handling and disposal.
On the other hand, water is a relatively reactive medium and most conventional catalysts would quickly become deactivated. To overcome this problem, several workers have conducted aqueous phase reactions over carbon-supported catalysts. For example, Olsen in U.S. Pat. No. 4,812,464 described certain aqueous phase hydrogenations over a palladium on carbon catalyst. Schwartz in U.S. Pat. No. 5,478,952 described the use of catalysts composed of highly dispersed ruthenium and rhenium on a carbon support for catalyzing aqueous phase hydrogenations.
Unlike many oxide supports, carbon supports can maintain their integrity under aqueous, acidic or basic reaction conditions. Carbon supports are also available with exceptionally high surface areas allowing preparation of catalysts with a high dispersion of active metals. Unfortunately, much of the surface area is contained within small micropores, which can result in severe mass transport limitations. As a consequence, many carbon supports are prepared as very fine powders to minimize mass transport effects due to pore diffusion limitations.
It has been found that many catalytic metals, once reduced, are not tightly bound to a carbon surface. During operation, these loosely bound catalytic metals can sinter, or agglomerate, thereby greatly reducing the available catalytic surface area. To lessen the effects of sintering, manufacturers typically limit the amount of catalytically-active metal to less than 1 weight percent of the total catalyst. While this results in a more efficient use of catalytic metal, it requires a larger bed to achieve the required conversions.
To improve properties of carbon-based catalysts, Heineke et al., in Canadian Patent No. 2,191,360, described certain carbon-based catalysts having a titania coating. In the invention of Heineke et al., a carbon support is treated with a Ti or Zr alkoxide, halide or mixed alkoxide-halide. Suitable carbon supports are generally suspended graphite or activated charcoal. Treatment of the carbon support with the Ti or Zr compound is preferably carried out in anhydrous solvents. The reaction is terminated by quenching with a small amount of water. The catalysts are then prepared by precipitating platinum in an aqueous medium using a reducing agent. In the examples, Heineke et al. suspended graphite particles in dry n-butanol and added titanium tetraisopropoxide or titanium tetrachloride followed by stirring for 2 days. Then water was added and the resulting particles are filtered off and dried. The resulting material was treated with an aqueous solution of hexachloroplatinic acid at a pH of 2.75. The platinum was precipitated by addition of sodium formate. The catalysts were tested in the hydrogenation of NO to NH2OH. Compared with catalysts lacking the titania layer, Heineke's catalysts showed better NH2OH selectivity (86.77 and 91.96 vs. 86.36 and 89.90) and space-time yield (0.798 and 0.897 vs. 0.788 and 0.870).
Despite these, and many other efforts, there remains a need for catalysts having new properties, especially catalysts that are stable in aqueous phase conditions. There also remains a need for new and cost-effective methods of making catalysts. There further remains a need for new aqueous phase catalytic reactions.